Directional desorption-driven nutrient fractionation enables product-oriented recovery from MBR-treated digestate by...

Recovering nutrients from real membrane bioreactor (MBR)-treated digestate remains challenging because strong ionic competition limits selective separation, while downstream product formation is highly sensitive to recovery-stream composition. Here, we developed an asymmetric membrane capacitive deionization (MCDI) platform that uses staged directional desorption to convert selective nutrient capture into controllable nutrient fractionation. A H₂O₂-oxidized biochar cathode enabled reversible NH₄⁺ storage at oxygenated carbon sites, whereas a biochar-supported ZnAl-NO₃ layered double hydroxide anode retained phosphate mainly through interlayer anion exchange, with a smaller, less labile fraction. During continuous operation, the system remained stable over 50 cycles, delivering average adsorption capacities of 53.02 mg g^-1 for NH₄⁺ and 37.32 mg g^-1 for phosphate while retaining >99% of anodic Zn and Al. Dual-stage directional desorption generated an NH₄⁺-enriched first-stage eluate and a phosphate-enriched, low-calcium second-stage eluate, with a phosphorus enrichment factor of 16.40. This controlled nutrient fractionation enabled struvite-dominated product formation with a 92.7% yield and without detectable secondary crystalline phases. Life-cycle assessment showed that the waste-derived, PV-battery-powered configuration reduced GWP₁₀₀ and AP by 32.7% and 91.9%, respectively, relative to the commercial biochar/grid-powered route, whereas EP was governed mainly by water and wastewater-related burdens. These results show that electrochemical nutrient recovery can be advanced from bulk ion removal to fractionation-enabled, product-oriented nutrient recovery in complex wastewaters.